There is technically one strain of SARS-CoV-2, the virus that causes COVID-19. What most people call “strains” are actually variants and sub-lineages of that single virus. Scientists have cataloged thousands of these genetic offshoots since the pandemic began, but only a handful have significantly changed the course of infections worldwide.
The distinction matters because it shapes how you think about immunity and vaccines. You’re not facing thousands of completely different viruses. You’re dealing with one virus that keeps tweaking itself, sometimes enough to cause a new wave of illness.
Why Scientists Say “Variants,” Not “Strains”
In virology, a “strain” refers to a virus that has changed so fundamentally that it behaves like a different organism, the way seasonal flu viruses differ from one another year to year. SARS-CoV-2 hasn’t crossed that threshold. Every version of the COVID virus, from the original detected in late 2019 through today’s circulating lineages, belongs to a single species and a single strain.
What the virus does constantly is mutate in smaller ways, producing variants. Some of these variants spread faster, dodge immunity more effectively, or cause different levels of illness. When a variant gains enough mutations to behave noticeably differently, health authorities flag it for closer monitoring. The World Health Organization sorts these into tiers: Variants of Concern (the most significant), Variants of Interest, and Variants Under Monitoring.
The Major Variants That Shaped the Pandemic
Out of the thousands of detected lineages, roughly a dozen variants drove major waves of infection globally. The ones most people remember by name are the five that WHO designated as Variants of Concern and gave Greek letter labels:
- Alpha (first detected late 2020, UK): Spread about 50% faster than the original virus and became the dominant variant in many countries by early 2021.
- Beta (late 2020, South Africa): Raised early alarms about immune evasion, meaning it was better at reinfecting people or reducing vaccine effectiveness.
- Gamma (late 2020, Brazil): Similar immune-evasion concerns, though it never became globally dominant.
- Delta (late 2020, India): Significantly more transmissible and caused severe illness in unvaccinated populations. It drove devastating waves in 2021.
- Omicron (late 2021, southern Africa): Carried an unusually high number of mutations and spread faster than any previous variant. It largely displaced Delta within weeks and has dominated global circulation ever since.
Each of these variants eventually faded as newer ones outcompeted them. Since late 2021, essentially all circulating COVID has been some descendant of Omicron.
What’s Circulating Now
The current landscape is entirely made up of Omicron sub-lineages. WHO updated its tracking system in March 2023 to reflect this reality, since every new variant of note is now an Omicron offshoot rather than something fundamentally new.
As of late 2024 and into 2025, the dominant sub-lineages are descendants of a branch called JN.1, which was first documented in August 2023 and designated a Variant of Interest in December of that year. Among hospitalized COVID patients in the U.S. whose virus was genetically sequenced between September and December 2024, the breakdown looked like this: about 52% had KP.3-like virus, 19% had XEC-like virus, 14% had KP.2-like virus, 8% had JN.1-like virus, and roughly 6% had other lineages.
These names (KP.2, KP.3, XEC) are all closely related branches on the same family tree. The differences between them are relatively small, which is why a single updated vaccine can offer meaningful protection against multiple circulating lineages at once.
How Vaccines Keep Up
Because the virus keeps evolving, COVID vaccines are now updated regularly, similar to the annual flu shot. The 2024-2025 COVID vaccines were formulated to target the KP.2 and JN.1 lineages. CDC data from the first few months of that vaccine’s rollout shows it provided about 33% effectiveness against COVID-related emergency department or urgent care visits within the first four months after vaccination. For adults 65 and older, effectiveness against hospitalization was higher, around 45-46% for those with healthy immune systems and about 40% for those with weakened immunity.
Those numbers may sound modest compared to the 90%-plus efficacy of the original vaccines in 2021, but the comparison is misleading. The original trials measured protection in a population with zero prior immunity. Today, nearly everyone has some baseline protection from previous infections, vaccinations, or both. The updated shot adds a measurable boost on top of that existing foundation, particularly against severe illness.
The fact that the newer XEC sub-lineage, which rose to nearly 20% of cases by late 2024, is closely related to the vaccine targets helps explain why protection held up even as the virus continued to shift.
Why the Number Keeps Growing
Every time SARS-CoV-2 copies itself inside a human cell, it can introduce small errors in its genetic code. Most of these mutations are meaningless. Occasionally, one helps the virus spread faster or slip past antibodies, giving that version an advantage. With billions of infections worldwide, the virus has had enormous opportunity to diversify.
Genomic surveillance databases have logged well over a thousand distinct Pango lineages (the naming system scientists use to track COVID’s family tree). New sub-lineages appear regularly. But the vast majority never become widespread. They circulate briefly in a small region and disappear, outcompeted by whatever variant is already dominant.
The practical number that matters to you is much smaller. At any given point in the pandemic, only one to three variants have driven the majority of infections globally. Right now, that means the JN.1 family of Omicron descendants. The thousands of cataloged lineages are scientifically important for tracking evolution, but they don’t represent thousands of meaningfully different viruses you need to worry about individually.